This application claims the priority under 35 U.S.C. xc2xa7 119 of Dutch Patent Application No. 1018086, filed May 16, 2001; the disclosure of this application is hereby incorporated by reference in its entirety.
1. Field of the Invention
The present invention relates in general to reactors for treating substrates. More particularly, the invention relates to reactors that process wafers and a method of operating such a reactor.
2. Description of the Related Art
When processing a substrate, such as a semiconductor wafer, the substrate is often heated with a heating means within a body of a furnace. It is customary that the energy supply to the heating means is controlled such that the measured temperature of the furnace body is substantially constant and has a desired value. When a number of substrates are subjected to a heat treatment one after the other, heat is withdrawn from the furnace body on the side of the furnace body adjacent to the substrate. Over the course of time a fall in the temperature of the furnace body will be detected by the temperature sensor. As a response to this, the energy supply to the heating means will be increased to such an extent that the furnace body again reaches the desired temperature. In view of the relatively high thermal capacity of the furnace body, this is a process that proceeds slowly and it can be some time before stable conditions have been established, in particular in those cases where the thermal capacity of the furnace body is so high and the treatment time so short that the temperature of the furnace body has still not been restored at the end of the treatment of a substrate. When a subsequent substrate has been loaded, heat is again withdrawn from the furnace body and in this way the temperature deviation can become increasingly greater for an initial number of substrates to be treated before it is finally restored as a result of the slow progression of the control process. The substrates subjected to treatment during this period will have received a non-uniform heat treatment.
These differences in heat treatment can be even greater than appears from the measured values produced by the temperature sensor. The temperature sensor is usually located within the furnace body, some distance away from the surface of the furnace body that is adjacent to the substrate. The heat, on the other hand, is withdrawn via the surface of the furnace body adjacent to the substrate. Decreases in the temperature of the furnace body of 10xc2x0 C. or more are possible at the surface or in the immediate vicinity of the surface. This is, of course, undesirable.
In the case of some heat treatments according to the prior art, after the substrate has been positioned in the vicinity of the furnace body, there is a waiting period until a stable, desired temperature has been established, after which the actual treatment, for example the deposition of a layer with the aid of plasma enhanced chemical vapor deposition, starts. However, in some heat treatments (e.g., annealing), the entire temperature-time-profile (i.e., the xe2x80x9cthermal budgetxe2x80x9d) plays an important role in the treatment, especially when the treatment temperature is higher than approximately 500xc2x0 C. Imposing a particular thermal budget on the substrates can even be the sole purpose of the treatment (e.g., without the formation of a layer on the substrate during the treatment). In such cases controlled and reproducible heating of the substrates is just as important as the final treatment temperature. In other words, it is important to achieve a thermal budget that is identical for all substrates when subjecting substrates to heat treatment. In principle, positioning the substrates in the vicinity of a relatively massive, heated furnace body is an extremely suitable method for this purpose, provided the disadvantages described above can be avoided.
A need therefore exists for a method and a device for heat treatment of substrates which avoid the disadvantages described above and which can achieve an identical heat treatment for successive substrates.
In one embodiment, a method for the successive heat treatment of a series of flat substrates comprises placing a substrates adjacent to, and essentially parallel to, a heating body having a flat boundary surface facing the substrate. The temperature in said heating body is measured at a location therein that is so close to the boundary surface that after the substrate has been placed in position the withdrawal of heat from the heating body by the substrate is measured at that location. The substrate is placed in the vicinity of said heating body, only after a desired temperature measured in the location has been reached. An amount of heat is supplied to said heating body such that the temperature measured at said location during the successive heat treatment of the series of substrates has an essentially constant value averaged over time. The substrate is removed from said heating body before said desired temperature, measured at said location, is reached again.
In this way an identical starting situation can be achieved for each substrate to be treated, which appreciably increases the reproducibility of the treatment. In a modified embodiment of the invention, after the substrate has been moved some distance away from the heated furnace body on completion of the heat treatment, it is moved into the vicinity of an essentially flat cooling body, so that cooling also takes place in a rapid and controlled manner.
The method described above can be carried out in various ways, in particular with regard to the control of the power supply to the heating means. For instance, it is possible in so-called xe2x80x9copen loop controlxe2x80x9d to supply a constant power to the heating means and, at the point in time when the treatment of substrates starts, to increase the power supply in order to compensate for the heat withdrawn from the furnace body by the substrates. It is also possible in xe2x80x9cclosed loop controlxe2x80x9d to control the power supply in such a way that the temperature measured by the temperature sensor is constant. Both in the case of xe2x80x9copen loop controlxe2x80x9d and in the case of xe2x80x9cclosed loop controlxe2x80x9d the control can be adjusted so that during the treatment of substrates the average temperature measured over time is somewhat higher than that in a state of rest. The result of this is that, following treatment and removal of a substrate, the temperature sensor indicates the desired temperature again within a shorter period and the introduction of the following substrate can start at an earlier point in time.
The method described above is particularly suitable for subjecting substrates to a heat treatment in a so-called xe2x80x9cfloating wafer reactor,xe2x80x9d as described in U.S. Pat. No. 6,183,565, which is hereby expressly incorporated by reference herein. In this floating wafer reactor flat substrates are brought one by one and successively between two essentially flat furnace bodies parallel to the substrate, after which the furnace bodies are moved towards one another and positioned a short distance away from the wafer. Preferably, the wafer is supported and held in place by gas streams, directed in opposing directions, issuing from these furnace bodies, without mechanical contact. With floating wafer reactors of this type it has proved possible to provide very rapid heating or cooling of the wafers without the wafers being damaged. As a result of the very rapid heating, wafers can also be treated very rapidly in succession.
According to another embodiment of the invention, substrates are treated in a treatment chamber in which, in addition to the heat treatment chamber, there is also a cooling station and a transport system for the substrate. The temperature of the substrate is lowered very rapidly in the cooling station under controlled conditions.
Yet another modified embodiment relates to a device for the heat treatment of a series of substrates. The device comprises a heating body with a flat surface for accommodating the substrate that is adjacent to the surface and controllable heating means for heating the heating body. At least one temperature sensor is positioned some distance away from the flat surface. The sensor is for measuring the temperature in the heating body and is connected to first control means for controlling the power supply to said heating means. Transport means are provided for positioning substrates in the vicinity adjacent to the heating body, holding them in this position and removing them therefrom. A second control means is provided for controlling the transport means. The temperature sensor is arranged near to the flat surface of the heating body in such a way that withdrawal of heat from the heating body by the substrate is detected. The first and second controllers are constructed such that moving each of the substrates into the heating body can take place only after a desired state has been reached. The desired state is determined by the temperature measured by the temperature sensor in the heating body as a function of time. The removal of each of said substrates takes place before said desired state has been achieved again.
According to an advantageous embodiment, the second controller comprises a low level control for controlling the motors of the transport means and a high level control for transmitting control signals to the low level control and receiving clearance signals. These clearance signals originate from sensors or from other control means and indicate that the device is ready for the next treatment action and that the device is in a safe condition for this further treatment action. One of these clearance signals originates, directly or indirectly, from the first controller for controlling the power supply to the heating means and is transmitted when the desired state has been reached within certain limits, determined on the basis of the signal transmitted by the said temperature sensor. The device can be provided with software to perform calculations on the signal transmitted by the temperature sensor in order to establish whether the desired state has been reached. This software can be operational in the first controller or a controller connected to this. When this clearance signal has been received and when any other requisite clearance signals are present, the high level control of the second control means gives a start signal to the low level control, which low level control causes the transport means to execute the desired movements. An unsafe condition of the device is determined by various sensors, such as sensors that indicate the position or presence of the substrate or of the transport means. If an unsafe condition is found while movements are being executed by the transport means, the high level control can transmit an interrupt to the low level control. As a result of this configuration, unspecified time delays in the substrate transport means as a consequence of checks carried out by the control are avoided and the transport of the substrates will take place as reproducibly as possible.
The heating means can be arranged in or on the furnace body or directly adjacent to the furnace body. It is also possible for the heating means to be arranged some distance away from the furnace body and for the heating means to comprise lamps or an induction coil.
All of these embodiments are intended to be within the scope of the invention herein disclosed. These and other embodiments of the present invention will become readily apparent to those skilled in the art from the following detailed description of the preferred embodiments having reference to the attached figures, the invention not being limited to any particular preferred embodiment(s) disclosed.
For purposes of summarizing the invention and the advantages achieved over the prior art, certain objects and advantages of the invention have been described herein above. Of course, it is to be understood that not necessarily all such objects or advantages may be achieved in accordance with any particular embodiment of the invention. Thus, for example, those skilled in the art will recognize that the invention may be embodied or carried out in a manner that achieves or optimizes one advantage or group of advantages as taught herein without necessarily achieving other objects or advantages as may be taught or suggested herein.